The present invention relates to the field of mass storage devices. More particularly, this invention relates to a method and apparatus for handling resonance effects in disc drives using active damping.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are an information storage disc that is rotated, an actuator that moves a transducer head to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so the data can be successfully retrieved from and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and accepting data from a requesting computer for storing to the disc.
The transducer head is typically placed on a small ceramic block, referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (xe2x80x9cABSxe2x80x9d) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the transducer head away from the disc. At the same time, the air rushing past the cavity or depression in the ABS produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider that is directed toward the disc surface. The various forces equilibrate so that the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically equal to the thickness of the air lubrication film. This film eliminates the friction and the resulting wear that would occur if the transducing head and the disc were to be in mechanical contact during the disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on the storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also said to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto the track by writing information representative of data onto the storage disc. Similarly, reading data from a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. Some disc drives have a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of drive. Servo feedback information is used to accurately locate the transducer head. The actuator assembly is moved to the required position and held very accurately during read or write operations using the servo information.
The actuator is rotatably attached to a shaft via a bearing cartridge which generally includes one or more sets of ball bearings. The shaft is attached to the base of the disc drive, and may also be attached to the top cover of the disc drive. A yoke is attached to the actuator. A voice coil is attached to the yoke at one end of the rotary actuator. The voice coil is part of a voice coil motor (VCM) used to rotate the actuator and the attached transducer(s). A permanent magnet is attached to the base and the cover of the disc drive. The VCM which drives the rotary actuator comprises the voice coil and the permanent magnet. The voice coil is attached to the rotary actuator and the permanent magnet is fixed on the base. The yoke is generally used to attach the permanent magnet to the base and to direct the flux of the permanent magnet. Since the voice coil sandwiched between the magnet and the yoke assembly is subjected to magnetic fields, electricity can be applied to the voice coil to drive the voice coil so as to position the transducer(s) at a target track.
Two of the ever constant goals of disc drive designers are to increase the data storage capacity of disc drives, and to decrease the amount of time needed to access the data. To increase storage capacity, current disc drives have increased numbers of tracks per inch (TPI). Put simply, current disc drives squeeze more tracks onto the same size disc. Decreasing the amount of time needed to access the data can be thought of as increasing the speed at which data is retrieved. Increasing the speed at which data is retrieved is very desirable. Any decreases in access time increase the speed at which a computer can perform operations on data. When a computer system is commanded to perform an operation on data that must be retrieved from disc, the time needed to retrieve the data from the disc is often the bottleneck in the operation. When data is accessed from a disc more quickly, more transactions can generally be handled by the computer in a particular unit of time.
A rotating disc data storage device uses a servo system to perform two basic operations: track seeking and track following. Track seeking refers to the ability of the disc drive and the servo system to move the read/write transducer head of the disc drive from an initial track to a target track from which data is to be read, or to which data is to be written. The settling of the transducer head at the target track is referred to as seek settling. Track following, which is performed after the head has been aligned with a target track, refers to the ability of the disc drive and the servo system to maintain the read/write head positioned over the target track. Note that, to effectively perform track seeking and track following in a disc drive with increased TPI, the servo open loop bandwidth of the system must also be pushed or increased.
Structural resonance in disc drives is one of the major challenges faced by disc drive designers in general, and disc drive servo control designers in particular. The structural resonance, such as arm bending mode resonance and coil bending mode resonance, will introduce problems in the operation of a disc drive""s VCM during seek settling and even during track following. Due to structural resonance, the position error signal (PES) for the actuator arm of a disc drive will oscillate during seek settling and track following, thus adversely affecting the settling time and the drive performance of the disc drive. The effects of structural resonance are getting worse with yearly increases in the number of TPI and in the servo bandwidth of the disc drives. As the number of TPI increases, the tracks become thinner and therefore it becomes crucial for the disc drives to minimize or eliminate resonance which can cause the actuator arm to swing to off-track positions when the actuator resonates during seek settling or track following. As the servo bandwidth increases, the susceptibility of the actuator to vibrations induced at the actuator""s resonant frequency increases, which may result in greater off-track disturbances of the heads.
While a certain amount of resonance is acceptable, the acceptable amount of resonance decreases as the number of TPI, and the servo bandwidth, increase. If the resonance is reduced or eliminated, the number of missed revolutions of the disc will be minimized and the access times will decrease. Reducing resonance will also help improve a disc drive""s through-put performance. If resonances in the actuator arm at frequencies associated with normal operation of the disc drive are reduced or eliminated, seek settling and track following will also be improved since the servo system will not be attempting to counter the effects of a resonating arm swinging across a desired track from an off-track position on one side to an off-track position on the other side of the desired track during the track settling or track following.
A common approach for addressing the structural resonance problem is to include an analog or digital notch filter to attenuate the resonant modes at particular frequencies. The control signal from the servo controller is passed through the notch filter before driving the VCM. Unfortunately, due to the nature of notch filters, the notch-filter approach introduces a large phase lag around the notch center frequency. Thus, while this approach is often used in disc drives when the resonance frequency is high compared to the servo bandwidth, the notch-filter approach is not applicable for handling resonance frequencies in disc drives that are near the servo open loop gain crossover frequency since the notch filter would cause an unacceptable phase margin drop. For example, the resonance frequencies of the arm bending mode and the coil bending mode resonance are about 700 Hz and 1000 Hz, respectively, which are both near the servo open loop gain crossover frequency. Thus, these structural resonance modes can cause problems in the track seeking and following operations which are not adequately addressed by the notch-filter approach. The seek settling and track following problems are worse if these frequencies coincide. Therefore, the notch-filter approach is not useful or adequate for handling certain resonance modes.
Therefore, what is needed is an improved method and apparatus for handling resonance effects in disc drives. There is also a need for a method and apparatus for handling resonance effects in disc drives which improves seek settling and/or track following in the disc drives, and may be used with disc drives that have increased numbers of TPI and increased servo bandwidths. There is also a need for a method and apparatus for handling structural resonance effects in disc drives that have resonance frequencies at or near the servo open loop gain crossover frequency of the disc drives. There is also a need for a method and apparatus for handling both high and low resonance frequencies in disc drives without loss of drive performance.
The present invention relates to a method and apparatus which use active damping for handling resonance effects in disc drives. Advantageously, this active damping approach can handle resonance effects which occur at either high or low frequencies without adversely affecting the performance of the disc drives. For example, the active damping approach of the present invention can handle structural resonance frequencies that may exist at or near the servo open loop gain crossover frequency of the disc drives, including both arm and coil bending mode resonances.
In accordance with one embodiment of the present invention, a method of handling a resonance effect on a disc drive includes the steps of monitoring a position error signal (PES) for an actuator arm of a disc drive, generating a feedforward compensation signal from the position error signal using a bandpass filter, and applying the feedforward compensation signal to a servo control signal. The bandpass filter has a center frequency that is set to a predetermined resonance frequency of the disc drive.
In one embodiment of this method, the monitoring step includes receiving a sensed position signal from a transducer coupled to the actuator arm, and subtracting the sensed position signal from a reference position signal. The bandpass filter has a center frequency that is set to a predetermined resonance frequency that appears during seek settling and/or track following. The generating step includes using a gain element with the bandpass filter to generate the feedforward compensation signal. The gain element provides a constant gain equal to the gain of the controller which generates the servo control signal at the predetermined resonance frequency. The applying step includes subtracting the feedforward compensation signal from the servo control signal, thereby achieving a phase advance in the open loop bode measurements. The method further includes providing the compensated servo control signal to an actuator assembly for actuating the actuator arm.
In accordance with another embodiment of the invention, a disc drive device includes a base, a disc rotatably attached to the base, an actuator assembly with an arm for carrying a transducer head in a transducing relation with respect to the disc in response to a control signal, and a controller coupled to the actuator assembly for monitoring a position error signal (PES) for the arm and for generating the control signal. The controller includes a servo controller that monitors the position error signal for the arm and generates a servo control signal from the position error signal, and also includes a feedforward compensation element, including a bandpass filter, that filters the position error signal to generate a compensation signal and combines the compensation signal with the servo control signal to generate the control signal. The bandpass filter has a center frequency set to a predetermined resonance frequency of the disc drive device.
In one embodiment of this disc drive device, the servo controller monitors the position error signal by receiving a sensed position signal from the transducer head, and subtracting the sensed position signal from a reference position signal. The bandpass filter has a center frequency that is set to a predetermined resonance frequency of the disc drive device which appears during seek settling and/or track following. The compensation element includes a gain element which provides a constant gain equal to the gain of the servo controller at the resonance frequency. The compensation element further includes a subtraction element for subtracting the compensation signal from the servo control signal to generate the control signal.
In accordance with another embodiment of the invention, an apparatus for handling a resonance effect on a disc drive includes means for monitoring a position error signal for an actuator arm of the disc drive, means for generating a feedforward compensation signal from the position error signal using a bandpass filter having a center frequency set to a predetermined resonance frequency of the disc drive, and means for applying the feedforward compensation signal to a servo control signal.
In accordance with another embodiment of the present invention, a disc drive includes a base, and an actuator arm and disc rotatably attached to the base. The arm carries a transducer head in a transducing relation with respect to the disc. The disc drive also includes means for handling a resonance effect using active damping.
These and various other features as well as advantages which characterize the present invention will be apparent to a person of ordinary skill in the art upon reading the following detailed description and reviewing the associated drawings.